Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/scanner/keyword.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of scanner;
 
 /**
  * A keyword in the Dart programming language.
  */
 class Keyword extends Iterable<int> implements SourceString {
@@ skipping 88 matching lines @@
 
   int get hashCode => syntax.hashCode;
 
   bool operator ==(other) {
     return other is SourceString && toString() == other.slowToString();
   }
 
   Iterator<int> get iterator => new StringCodeIterator(syntax);
 
   void printOn(StringBuffer sb) {
-    sb.add(syntax);
+    sb.write(syntax);
   }
 
   String toString() => syntax;
   String slowToString() => syntax;
   String get stringValue => syntax;
 
   SourceString copyWithoutQuotes(int initial, int terminal) {
     // TODO(lrn): consider remodelling to avoid having this method in keywords.
     return this;
   }
@@ skipping 76 matching lines @@
 
   ArrayKeywordState(List<KeywordState> this.table, String syntax)
     : super((syntax == null) ? null : Keyword.keywords[syntax]);
 
   bool isLeaf() => false;
 
   KeywordState next(int c) => table[c - $a];
 
   String toString() {
     StringBuffer sb = new StringBuffer();
-    sb.add("[");
+    sb.write("[");
     if (keyword != null) {
-      sb.add("*");
-      sb.add(keyword);
-      sb.add(" ");
+      sb.write("*");
+      sb.write(keyword);
+      sb.write(" ");
     }
     List<KeywordState> foo = table;
     for (int i = 0; i < foo.length; i++) {
       if (foo[i] != null) {
-        sb.add("${new String.fromCharCodes([i + $a])}: ${foo[i]}; ");
+        sb.write("${new String.fromCharCodes([i + $a])}: ${foo[i]}; ");
       }
     }
-    sb.add("]");
+    sb.write("]");
     return sb.toString();
   }
 }
 
 /**
  * A state that has no outgoing transitions.
  */
 class LeafKeywordState extends KeywordState {
   LeafKeywordState(String syntax) : super(Keyword.keywords[syntax]);
 
   bool isLeaf() => true;
 
   KeywordState next(int c) => null;
 
   String toString() => keyword.syntax;
 }